Interrelated pulp-refining and viscose process



Dec. 20, 1927.

Filed Oct. 29

G A RICHTER INTERRELATED PULP REFINING AND vlscos:` PROCESS Patented Dec. 20, 1927.

UNITED 4STATES PATENT OFFICE.

` GEORGE A. RICHTER, OF BERLIN, NEW HAMPSHIRE, ASSIGNOR TO BROWN COMPANY,

F BERLIN, NEW HAMPSHIRE, A CORPORATION. OF MAINE.

INTERRELATEI) PULP-REFINING .AND VISCOSE PROCESS.

Application led October 29, 1925. Serial No. 65,835.

This invention has relation to the production of viscose from cellulosic fibrous material. More particularly, it concerns itself with the production of viscose from high 5 alpha cellulose fiber: that is, a pulp containinol a high percentage of alpha orfresistant cel ulose. i

The usual proc ss employed in the preparation of viscose from cellulose ulp consists in subjecting air-dry pulp 'in s eet form (e. g., sulphite pulp) to a bath of caustic soda solution approximating 18% in concentration of caustic soda. After the sheets have been thoroughly impregnated or sati5 urated with the caustic soda solution, they are removed from the bath and subjected to pressure in a manner to express or remove the excess caustic soda solution contained therein but to allow sufficient caustic soda o` to remain behind for efectin the yconversion of the cellulose fiber into t ie soda cellulose, so called. The pressed pulp sheets with the caustic soda solution contained therein, are allowed to stand or age for two days or so, during which time a reaction takes place between the caustic soda and cellulose. There is some question regarding the precise reaction consummatedbetween the reacting components, but it is generally assumed that the cellulose forms an addition product with the caustic soda.

The soda cellulose or addition product thus produced is then treated, preferably While agitated with the amount of carbon bisulphide necessary to form cellulose Xanthate or viscose. Enough additional caustic soda and water are added during or after the completion of the reaction to permit the solution of cellulose xanthate formed by the reaction in the alkaline liquor. The result ing colloidal product may be a yellow, orange, red or greenish syrup or jell, depending upon the impurities contained in the ingredients and the physical conditions attending the reactions.

The colloidal syrup or jell thus prepared is allowed to stand or ripen to the proper degree. The ripening step requires two or three days, during which time certain spontaneous chemical reactions or decompositions set in, in the viscose, which alter its physical and chemical properties and condition it for spinnno and setting into a silk-like fiber. After the viscose has been properly ripened or conditioned, it is spun b 'extruding or 55 squirting 1t under ressure t rough the orifices of speclally esigned and constructed spinnerets, and 1s set-as a continuous strand or filament in a precipitating or setting bath, which usual y contains niter cake (NaHSQ), salt cake (Na,SO,), and free sulphuric acid. The wet precipitated filament may be wound on a spool or equivalent winding device, from which it may be unwound forV subsequent treatment, suchas washing, bleaching, twisting, or other operations carried out in connection with or insident to the preparation of thread.

The physical characteristics of the finished silk-like or artificial silk filament or thread thus produced are in great measure undoubtedly dependent upon the characteristics of the original cellulosic fiber employed as the raw material. For example, it is impossible to manufacture artificial silk of substantial or 'workable strength from ground wood, cheapsoda pulp, or the usual unbleached kraft pulp. Even the ordinary unbleached sulphite pnl is unsuitable for such manufacture, an results in an inferior product which may be commercially worthless. While certain hi h grades of bleached sulphite pulp may e employed in the manufacture o artificial silk, the' resulting product is usually inferior to one resulting from' 4the employment of cotton linters as the raw cellulosic fibrous material; in other words, the strength and other desirable hysical qualities in an artificial silk pro uct are oubtless dependent upon the strength'and 9o alpha cellulose content of the cellulosic bers or raw material from which it is prepared. The purified or high alpha cellulose ulp prepared and utilized in the practice of) the present invention for the preparation of the viscose may or may not possess papermaking characteristics, dependin upon the strength of the caustic soda so ution employed and the various conditions under which the purification treatment is effected, in contradistinction to the special conditions under which the purification reaction must be carried out Where a substantially unmercerized high alpha cellulose fiber suitable as a rag substitute in papermaking is desired. In my applications, Serial No. 26,361, filed April 28, 1925, and Serial No. 52,232, filed August 24, 1925, there are described meth- 4' in purifying the unbleached pulp to pr ods for purifying pulp, and more especiall sulphite ulp, for the production of a hiv alpha cel ulose product especially suitable or conversion into cellulose derivatives. The method described in the first mentioned atplication consists in treating unbleached su phite pulp at an elevated temperature with a relatively dilute solution of caustic soda for a sufficient period of time to eliminate the ligneous and less-resistant celluloses contained therein, resultin `in a product high in resistant or al ha cel ulose. The urified product may be leached to high w iteness without the necessity of an excessive amount ofbleach and Without affecting the strength oralpha cellulose content thereof, or otherwise injuring it appreciably.

The method described in the last mentioned application consists in treating pulp, and more especially unbleached sulphite pulp, at ordinary or room temperature, with a relatively concentrated solution of caustic soda containing sufficient alkali to react with and remove less-resistant celluloses contained in the pulp, but insufficient to cause a substantial mercerization of the alpha cellulose contentthereof, thereby yieldlng a product notably high in alpha cellulose. The product `thus obtained is easily bleachable to high whiteness with a moderate amount of bleach, without substantially affecting its other properties.

It is thus to be observed that the same reagent viz, caustic soda, may be empcyed uce a high alpha cellulose fiber especially suitable for conversion into a celluloze derlvative such as viscose, and for converting the alpha cellulose into the soda cellulose, which constitutes the initial step of the viscose process.

Ordinarily a portion of the caustic soda liquor expressed from the pulp sheets in forming the soda cellulose is later converted lnto a sulphate solution which is employed in the setting bath. Such practice is ollowed because the liquor expressed from the sheet contains certain organic impurities dissolved from the pulp, and the re-use of all such liquor for converting other sheets of pulp to soda cellulose would cause the impurities to become more highly concentrated, although the solution may be re-used, after the addition thereto of sufiicient caustic soda to convert such other pulp into soda cellulose and to make up losses in caustic soda resulting from its previous use; however, the continued re-use of and treatment with such a solution would ultimatel not permit the purification incident to such treatment by the use of a fresh solution of caustic soda, since a condition of dynamic equilibrium is reached when the pressed sheets contain the amount of impurities contained therein before such treatment. Hence a substantially fresh caustic soda solution is preferably employed in treating the pulp sheets for conversion into soda cellulose.

In accordance with the present invention, where the preparation of the viscose occurs simultaneously or in connection with the preparation of a high alpha cellulose prod'- uct for use in making such viscose, the caustic soda solution expressed from the sheets of pulp may be advantageously employed in purifylnfr other pulp to produce the hi h alpha cellulose fiber either by a hot or co d treatment and the purified pulp is treated with a su stantially fresh solution of caustic soda.

With the foregoing and other features in view, a more complete understanding of the present invention may be gained from the following description thereof when considered in conjunction with the accompanying drawings, which may be termed a flow sheet, and which shows the sequence of treatments and the progress of flow of the various materials.

Referring to the drawings, at 1, I have indicated a storage tank containin a supply of pulp, preflerably an unbleaied sulphite pulp produced by the liberation of fiber under the proper conditions from wood chips (e. g., spruce) in a suitable acid sulphite liquor, and which is to undergo a purifying or refining treatment with a caustic soda solution preparatory to its conversion into artificial silk by the viscose process. The pulp is delivered to a set of pulp thickeners 2, wherein the pulp is thickened to the desired consistency. The thickened pulp is then mixed with a sufficient amount of caustic soda liquor expressed from the sheets of previously purified or alpha pulp 1n the pressing operation. The liquor expressed from the purified sheets in the viscose process is thus utilized to effect initial treatment or purification of the unbleached pulp. The pulp together with the solution of caustic soda admixed therewith is delivered to a reaction tank 3 communicating at its lower portion by a slow-moving worm with a second reaction tank 4.

Where the pulp is to be purified by a cold treatment, the stock undergoing purification in the tanks is Apreferably maintained at a stock consistency of about 5% to 6%, the treating liquor containing about 100% to 150% caustic soda based on the dry weight of fiber. By regulating the degree of thickeninn' and the amount of expressed.' liquor admixed therewith, such stock and caustic soda concentration may be produced. The purifying treatment is carried out preferably while stirring or agitating the pulp so as to promote a uniform reaction at room temperature (say, 30 C.), and the purification reaction may require 30 minutes to -two hours, depending on the characteristics of the purification of the pulp may be effected at au elevated temperature 1n a relatively dilute solution containing about 5% to 6% of caustic soda.

The resulting purified product, in either case, usually contains from about 911% to 98% alpha cellulose, and may be washed substantially free from entrained reaction products and from alkali, in a counter-current washer 5. The spent liquor which leaves the counter-current washer and is delivered to a storage tank not only conta-ins a very high percentage of residual unconsumed caustic soda, but the various organic substances removed from the original pulp. The liquor will vary from a light brown to a dark brown color, depending upon the `character of the original raw fiber stock. A large portionv of such spent liquor may be re-employed, as indicated on the drawing, for admixture with and in the treatment or purification of other raw sulphite pulp, and a portion ma be withdrawn and the chemicals contain'e therein may be recovered as in' connection with the hot process described in the application-previously referred to. The diverted or withdrawn portion of spent liquor, however, may be converted into salt cake, by treatment with sulphuric acid, if desired, and may be employed in the preparation of the setting bath.

The washed alpha cellulose fiber is preferably bleached before conversion into viscose. To this end, the purified stock is passed rinto a storage tank 7 from which it is withdrawn and supplied to a set of thickeners 8, wherein it is concentrated to the consistency desired for bleaching. The concentrated stock from the thickeners is di.- luted to the desiredconsistency, say about 10% to 16%, with the required amount of bleach liquor delivered from a supply tank 9, and is passed into bleaching tanks 10 and 11, arranged -in series, wherein the stock is bleached while agitated and at the required temperature for the necessary period oftime. 'I he amount of bleach required in the bleaching operation depends upon the characteristies of the initial raw vpulp stock and the degree of purification edected by the alkaline treatment withxcaustic soda. Under ordinary conditions, about5% to 8% of bleach,

to bring the stock up to the desired degree of whiteness. The stock after bleaching is delivered to a multiple washing machine 12, preferably of the NVolf type, and is washed substantially free of bleach solution and entrained soluble reaction products prior o sheeting and conversion into soda celluose.

The bleached, purified pulp, containing a high percentage of alpha cellulose, is passed into astorage tank 13, from which it is supplied toa wet or cylinder machine 14 and formed into-sheets and dried to a. water cony tent of about 10%. The sheets of purified pulp which are new prepared for conversion into soda cellulose are delivered to a soaking I bath 15, wherein they are immersed in va solution containing 16% to 20% caustic soda,

-until they are completely saturated therewith. lThe soaked sheets are then forwarded to a suitablepress 16, by means of which the excess caustic soda liquor is expressed therefrom, but sufficient caustic soda is left behind within the sheets for their conversion into soda cellulose. The sheets are then aged under proper conditions in a storerooni 17, until the cellulose has undergone the necesjsary chemical change and has been converted into a soda cellulose, whereupon the soda cellulose vthus formed is subjected to the other treatments previously alluded to as being necessary for the manufacture of finished artificial silk filament.

The new caustic soda which is necessary for replacing sodium losses in the entire -purification cycle, as well as the sodium constituent accompanying the pressed sheets of purified pulp for the viscose process, is added to the soaking vat 15, provision being made to add sufficient caustic soda to the vat to maintain a liquor containing 16% to 20% caustic soda. The caustic soda is preferably added in the forni of a strong solution to replace the corresponding volume of solution removed therefrom by the sheets of purified pulp. Due to reaction between the caustic soda solution and the small quantity of less stable celluloses contained in the purified pulp, as well as to the adsorption of a certain amount of caustic soda from the solution by the sheets, the caustic soda liquor expressed from the pulp and passed into Y the storage tank 18 contains only about 15% to 18% of active caustic soda. As previously indicated and shown on the drawing, this expressed liquor is utilized in accordance with the present invention for admixture with the raw pulp to be purified, as it is fairly free from contamination, and possesses sufficientV unreacted or free caustic soda to be used for the initial purification of the raw pul V At times, all the press liquor available from the pressing operation may be necessary in the treatment of the raw pulp to be purified. Under certain conditions of operation, however, it may occur that there is more i prcss li uoi` available than necessary for e cting t e purification of the' raw or unrefinedpulp. Thisvmay especially be the case where a. hot treatment with a. dilute solution of caustic soda is employed foreffecting such purification. In such cases, the excess portion of the press liquor which is not needed in the purification treatment ma be returned to the soaking vat, as indicate in which case the volume of caustic soda solution supplied thereto at a somewhat higher concentration will correspond substantially to the volume removed therefrom by the sheeted purified pulp minus that portion which is returned or re-einployed from the pressing operation; or the excess portion may be diverted for use in the setting bath after conversion-finto salt cake; or a procedure may be followed wherein a fraction of the excess press liquor is employed for the preparation of the setting bath and theremaining fraction is re-employed vin the soaking of the purified sheets of pulp, and the production of a high grade artificial silk product or filament is ensured. Such an artificial silk is of notably high strength as compared to the artificial silk product obtainable from the usual untreated pulps by the viscose process, and hence compares more favorably with a product made from cotton linters.

While I have confined the description of the process of thisinvention as exemplified by the use of unbleached sulphite pulp as the raw material, it is equally applicable in processes wherein other pulps such as kraft or certain grades of soda pulp are first Subjected to a purification treatment with caustic soda, and the pulp thus treated is subsequently converted into cellulose xanthatc or viscose, as herein described.

Before concluding, it may be stated that by employing a purification treatment of raw pulp wherein a relatively concentrated solution of caustic soda is employed at low temperature, a somewhat larger alpha cellulose content, and a smaller yield, which is reduced relatively to a greater extent than the increase in alpha cellulose, is obtained than when such pulp is urified with a relatively dilute solution ofp caustic soda at an elevated tem erature. The characteristics of the purifie or high alpha cellulose pulp depend upon the type of treatment employed, the cold treatment with a relativel concentrated solution of caustic soda yield ing a purified pulp which is notably low in pentosaii content and which is generally more suitable for conversion into cellulose xant-hate. However, where the entosan content is immaterial in certain gra es of viscose, the hot treatment with a relatively dihte olution of caustic soda may be emp o e 4 v aving thus described this invention, it should be obvious that various changes and modifications might be resorted to in the procedure without departing from the spirit or scope of invention as defined by the appende claims.

What I claim is: 85

1. An interrelated pulp-purif-yln and viscose process which comprises t e following steps, to-wit: treating pulp with a solution containing caustic soda, to effect the purification tl1ereof ;;washing the purified pulp; sheeting said pulp, soaking the sheeted'mate'rial withv a solutionof caustic soda preparatory to conversion into viscose; expressing the excess caustic soda solution from said sheets; and employin such expressed liquor' in said first mentioned step for the like treatment of other pulp.

2. AA cyclic process of treating pulp in the production of viscose, which comprises soaking purified pulp in a bath comprising a solution of caustic soda, removing the pulp therefrom and ex ressing excess causticsoda solution therefiom; and purifyin raw pulp 'for such soaking treatment by sai expressed caustic` soda solution.

3. A. cyclic process of treating pulp in the production of viscose, which comprises continuously feeding a stream of pulp through and soaking it in a bath of caustic soda solution, expressing excess solution therefrom, continuously supplying the expressed solution to the stream of ulp prior to its being fed to the soaking bat to purify such pul continuously washing such stream of pulp after reaction with such expressed solution and prior to its being fed to the aforesaid bath, and supplying fresh caustic soda solution to said bath to make up losses therein.

4. In the manufacture of viscose, a process which com rises feeding a mass of pulp, previously puried by treatment with a solution containing caustic soda, to and soaking it in a bath of caustic soda solution preparatory to its conversion into viscose; continuously expressing the excess caustic'soda 125 solution from said mass but allowing Sullicient caustic soda to remain therein for its conversion into soda cellulose; supplying a portion of the expressed excess solution to a mass of pulp for the purification thereof .130

prior to its being fed to such bath, and returnin av portion of such Iexpressed solution to sai bath.

5. A process which comprises soaking a mass of pulp, previously purified by treatment with a solution containing caustic soda, in a bath of caustic soda solution preparatory to its conversion into viscose; removing said mass from said bath and expressinc theexcess caustic soda solution from said mass but allowing sufficient caustic soda to remain thereinfor its conversion into soda cellulose; treating raw pulp with such expressedsolution for the purification thereof prior to its being soaked in said bath; and adding caustic soda to said bath sufficient to make up sodium losses Atherein due to the purification of such pulp and to the removal therefrom of a portion of the caustic soda necessar for the conversion of said pulp to soda cel ulose.

6. An interrelated pulp-purifying and viscose process which comprises the followin steps, to Wit: feeding a stream of raw pu p and a solution containing caustic soda through a reaction chamber to effect the purifica-tion of such pulp; washing the purified pulp delivered therefrom; sheeting said pulp; soaking the sheeted material in a ath of caustic sodal preparatory to conversion into viscose; removing the sheets from the bath and expressing the excess caustic soda solution therefrom, and circulating such expressed solution back to the reaction chamber for the purification of the stream of raw pulp.

7 An interrelated pulp-purifying and viscose process which comprises the following steps, to Wit: digesting ravv pulp in a bath of caustic soda solution to effect t-he purification of such pulp; Washing the puried pulp; sheeting saidpulp; soaking the sheeted material in a bath of caustic soda solution preparatory to its conversion into viscose; removing the sheets from the bath and expressing the excess caustic soda solution therefrom; and circulating a portion of such expressed solution to said first ment-ioned purifying bath'and circulating the remaining portion to the soaking bath.

8. An interrelated pulp-purifying and viscose process which comprises digesting raw pulp in a bath of caustic soda solution to effect a purification of such pulp; washing the purified pulp; sheeting such' pulp; soaking such sheeted material in a solution of caustic soda preparatory to its conversion into viscose; removin the sheets from the bath and expressing t e excess caustic soda solution from said sheets but allowing sufficient caustic soda to remain therein for its conversion into soda cellulose; circulating such expressed solution to said first mentioned purifying bath, and adding sufficient caustic soda to said soakin bath to make up sodium losses incident to t e purification of such pulp and to the removal therefrom-of a portion of the caustic soda necessary for the conversion of said pulp to soda cel ulose.

` 9. An interrelated pulp-purifying and viscose process which comprises digesting raw pulp in a bath of caustic soda solution to effect a purification of such pulp; separating the purified ulp from the solution; reemploying the soliition for the digestion of other pulp; sheeting the purified pulp; soaking such sheeted material in a bath 0f caustic soda. solution preparatory to xanthation; removing the sheets from the bath and expressing the excess caustic soda solution from said sheets; circulating such expressed solution to said first-mentioned purifying bath; and adding sufficient caustic soda to said soaking bath to make up sodium losses incident to the purification of such pulp and to t-he removal therefrom of a portion of the caustic soda necessary for xanthation.

10. An interrelated pulp-purifying and viscose process which comprises digesting raw pulp in a bath of caustic soda solution to effect a purification of such pul separating the purified pulp from the so ution; reemploying a portion of the solution for digestion of other pulp and employing a portion for the viscose setting bath; sheeting the purified pulp; soaking such sheeted material in a bath of caustic sodal solution preparatory to its xanthation, removing the sheets from the bath and expressing the excess caustic soda solution from said sheets; circulatin such expressed liquor to said first mentioned purifying bath; and adding sufficient caustic soda to said soaking bath to make up sodium losses incident to the purification of such pulp and to the removal therefrom of a portion of the caustic soda necessary for xanthat-ion.

11. An interrelated pulp-purifying and viscose process which comprises digesting raw pulp in a bath of caustic soda solution to Yeffect a purification of such pulp; separating the purified pulp from the solution; re-employin the solution for the di estion of other pu p; bleachin the` urfie pulp and Washing the bleache pro uct; sheetiner the bleached and washed pulp; soakin sai sheets in a bath of caustic soda so ution preparatory to its xanthation; removing the sheets from the bath and expressing the excessl caustic soda liquor from said sheets, and circulating such ex ressed solution to said first-mentioned purifying bath.

In testimony whereof I have affixed my signature.

GEORGE A. RICHTER. 

